This invention relates to electromagnetic flowmeters and in particular to such meters in which the flow of fluid through the meter may be in either of two opposite directions.
In an electromagnetic flowmeter a magnetic field across a flow tube generates a voltage in a fluid flowing through the tube. The voltage, which is proportional to flow rate, is sensed by a pair of electrodes in contact with the fluid, and is amplified by a differential amplifier to produce a flow-dependent signal which is amplified by a signal processing system to produce an output signal which is proportional to flow rate. When the flow is reversed in the flow tube, the voltage, hence the flow-dependent signal, is also reversed. If the signal processing system has the capability to handle signals of both positive and negative polarity, as does a microprocessor-based system which produces positive or negative "words" indicative of both the polarity and magnitude of an input signal, the output signal is indicative of both flow rate and direction without further signal processing. Unfortunately, most commonly used magnetic flowmeter signal processing systems do not easily and accurately handle signals of both positive and negative polarity. It has therefore recently been proposed to add to a conventional magnetic flowmeter signal processing system a "negative one" amplifier, which when switched into the system inverts the polarity of the flow-dependent signal. This system, described in U.S. Pat. No. 4,089,219 to Suzuki, adds a component (the negative one amplifier) to the signal processing chain during reverse flow conditions. It therefore requires that the negative one amplifier invert the signal accurately and without any offset error. These requirements are difficult to achieve, and generally require an additional adjustment to ensure accurate calibration of the reverse flow measuring system.